1. Field of the Invention
This invention relates generally to magnetic recording sensors, particularly to methods of pinning anti-ferromagnetic layers in multiple directions.
2. Description of the Related Art
The state of the art GMR (giant magneto-resistive) and TMR (tunneling magneto-resistive) magneto-resistive sensors produce a resistance change in response to an external magnetic field applied to them. The conventional film structure (configuration of magnetic and non-magnetic layers) of a GMR or TMR sensor is shown in (prior art) FIG. 1. Typically, the film structure includes an anti-ferromagnetic (AFM) layer (1), a ferromagnetic pinned layer (PL) (2), a non-magnetic Ru layer (3), a ferromagnetic reference layer (RL) (4), a non-magnetic barrier layer (5) and a ferromagnetic free layer (FL) (6). The AFM layer exchange couples to the PL layer. Due to the AFM layer's lattice structure, the PL magnetization is pinned in the +y direction as shown by arrow (11). The combined PL/Ru/RL then forms a synthetic anti-ferromagnetic (SAF) structure (9), which aligns the RL magnetization (12) in the opposite direction to the magnetization of the PL (11). Thus the combined AFM/PL/Ru/RL forms a film stack that keeps the magnetization direction (12) of the top RL layer from changing direction in the presence of an external magnetic field. The RL (4), the barrier layer (5) and the FL (6) then forms the GMR or TMR junction, where the barrier layer (5) is typically a layer of Cu (a non-magnetic conductor) for the GMR or a layer of metal oxide (a non-magnetic insulator) for the TMR. The resistance across this junction changes in accord with the relative angle of magnetization between the RL and FL magnetizations. The FL magnetization can be rotated by an external magnetic field. Since the magnetization direction of the RL is fixed, the relative angle and, therefore, the resistance, depends only on the magnetization direction of the FL.
For a GMR or TMR sensor to operate as described above, a pinning anneal process step must occur during sensor fabrication. In this process step, the sensor is heated above the blocking temperature of the AFM material so that the AFM layer changes from its AFM phase to a non-AFM phase. Meanwhile, a significantly strong external magnetic field is applied to the sensor in the direction of the desired magnetization of the PL, so that the magnetization of all magnetic layers in the sensor stack, including PL, RL and FL, are aligned in this strong field direction. Then the sensor is cooled down to room temperature for the AFM layer to return to the AFM phase through lattice reordering, while the externally applied strong magnetic field still holds. After the field is removed, the AFM layer maintains its memory of the field direction with the newly formed lattice structure and also pins the magnetization of the PL in the direction of this strong field through a surface exchange interaction. The magnetization direction of the RL (12), on the other hand, reverses direction due to the SAF effect, so it is now anti-parallel to the direction of the PL magnetization (11).
In the conventional MR (GMR or TMR) sensor fabrication process, the MR sensors on a single wafer usually have the same PL magnetization direction that results from a single step pin anneal. However, in some application areas, two or more different pin directions of magnetization are needed. In the prior art, Maiwald (U.S. Pat. No. 6,882,146 B2) discusses the magnetic angle sensor application using GMR sensors where both orthogonal and opposite pin directions are used to measure the arbitrary angle of an externally applied magnetic field. This application requires the fabrication, on a single wafer, of sensors with different pin directions.
For MR sensors having different AFM pin directions at different locations on the same wafer, the laser heating method described by Adachi et al. (U.S. Pat. No. 6,727,125) may be utilized. In this prior art, a shadow mask is used to allow laser light to illuminate and heat specific regions of a single wafer at a given time. Therefore, different regions can be annealed at different times with different magnetic field directions, which will determine the AFM pin directions at these locations. One limitation of using this method to directly heat an MR sensor element to achieve multi-directional AFM pinning, is that conventional materials used in the MR fabrication process have poor light absorption efficiency. For example, Wei et al. (U.S. Pat. No. 7,495,230) disclose a method for detecting plasmons generated by a laser that requires an integrated structure that first generates polarons that are subsequently converted to thermal electrons.
To achieve sufficient heating of the AFM material, either very high laser power or excellent focusing of the laser light will be required. It is desirable to have a high efficiency light absorbing material that can absorb the light energy and heat the AFM material effectively at a desired location, so that it either relieves one of the need for a high power light source and expensive optics or, at the same optical power, provides an increase in wafer throughput.